I did the same many years ago when I lived in Seattle.
It was the best trading collaboration I ever had...something more useful than stay at home trading...both of which I've done. Its also something I plan to return to when my kids become teenagers (more independent) and I've already located a few office locations for such.
Anyways, I highly recommend you better advertise this with several viewable pictures from inside the office along with mentioning nearby stores or leisure activities without giving any specific names just in case you don't want anonymous strangers from ET showing up unannounced (without appointment).
For example, if there's a gym in the building or down the street...mention such without naming the gym. If there's book store nearby...mention that without naming the store. If you're close to the subway or parking conditions...mention that without naming these places. Does the office have its own bathrooms or will the trader need to leave the office and go to the bathroom down the hall or go to another floor ?
Also, you said something odd...you said
no rent. Therefore, you should mention if there's other fees involved or requirements and if you're supplying the computer equipment or if such is the responsibility of the trader.
Seriously, when I first did it...I did it very similar to what you just did and got only 3 interests. Then I did it the way I just recommended it and I got +20 interests when I was only looking for 5 traders. Simply, you need to sell this place as being cool especially if you're in the New York area.
Good luck and wish you the best.
P.S. Be prepare for odd but understandable requests like wanting a desk near a window...a window that can open to let in fresh air on days when the air quality in the office sucks...such as when someone releases an atomic fart.